The invention is related to the use of flexible production and water injection risers and control umbilicals with offshore structures and more particularly to a riser disconnect and support mechanism.
Floating offshore structures used in drilling for and production of hydrocarbons (natural gas and oil) use drilling and production risers that typically extend from the sea floor to the keel of the structure and then to the topside of floating structures.
A potential hazard in offshore operations is the escape of hydrocarbons and other products from the production risers and control umbilicals into enclosed locations in and around the facility structure. These hazards may be caused by damaged risers or failures in mechanical connectors in the flow lines inside the facility.
In some situations the riser arrangements may have to be disconnected from the supporting facility and this facility returned for reconnect at a later time. For example, offshore structure designs for deployment in arctic regions have to consider ice forces that can be the governing design load. Unlike bottom founded structures such as compliant towers and jackets and gravity base structures (GBS), floating structures are challenged by mooring and riser designs that make resistance to maximum expected ice loads impractical and thus require disconnection from the risers and moorings as part of the ice management scheme. Also the floating support hull may be returned to port for refitting or reconfiguration of the topsides.
Moored floating structures such as the ship-shaped Floating Production Unit (FPU), the Spar, and the Single Column Floater are practical designs for support facilities. Even in shallower water where earthquakes are a threat, the moored floater can be the better option because of its ability to avoid seismic effects of an earthquake on the structure since it is suspended in the water above the sea floor.
Several designs to disconnect and support riser arrangements from the floating support facilities presently exist.
The FPSO/PPS (Floating Production Storage and Offloading/Floating Production and Storage) generally has a weather-vaning mooring turret attached inboard at the keel. Risers and umbilicals pass through the turret up to the onboard production facilities. For disconnect between the risers and hull, the risers are disconnected at the turret and released to separate from the hull. After release the buoy is suspended in the water column with the aid of mooring lines and supports the risers. To reconnect, the buoy is recovered by the hull and pulled back into position. The risers are reconnected at the turret. The draft of the ship-shaped hull is generally in the order of 30 meters. At this draft it is practical to provide one atmosphere dry access to the assembly around the turret to make it accessible for inspection, maintenance, and repair.
Other designs based on deeper draft facilities such as the Spar and Single Column Floater have drafts in the order of 100 meters to 200 meters. These hull types offer the advantage of reduced motions, thus improving conditions for general operations and have a significant reduction in fatigue damage to the risers as compared to the shallower draft ship-shaped hulls. Spar based designs such as U.S. Pat. Nos. 7,377,225 and 7,197,999 describe disconnectable buoys at the keel similar to the FPSO/FPU with riser disconnect at the keel. The disadvantage of these designs is the depth of the disconnect buoy. Due to the in-situ pressure and space constraints inspection, maintenance, and repair are difficult and complicated. There is also risk that hazardous product escaping from the risers due to faulty connections at the buoy can collect inside the hull.
Floating offshore structures with relatively low clearance between the bottom of the structure and the sea floor also present special challenges for the connection and disconnection of risers at the bottom or sides of the structures. The flexible risers typically used with floating offshore structures have a minimum allowable bend radius beyond which will cause breakage of the riser. Also, the flexible risers must not touch the sea floor during connection to or disconnection from the structure and during the time that the risers are supported when not connected to a structure. These two challenges are not satisfactorily addressed in the current art.